As is known, efficiency of a fuel cell power plant begins with the degree to which all of the fuel provided to the power plant is utilized in production of electricity. This is called overall fuel utilization. It is desirable to achieve fuel utilizations in excess of 98%. Providing exactly the amount of fuel which is required for the instantaneous load on the fuel cell power plant cannot be achieved in fuel cells utilizing current technology due to local fuel starvation at various parts of individual cells and among various cells of a fuel cell stack, which in turn is caused by maldistribution of fuel. Maldistribution of fuel to various cells is caused by inadequacies of fuel manifolds, and distribution variations within a given cell are caused by channel depth variations in the fuel flow field. If a multi-pass fuel distribution system is used, in which a portion of all cells receive fuel traveling in one direction through the stack, after which the fuel is turned so that it passes through the other half-portion of all of the cells, variations in leakage at the turn manifold cause maldistribution variations. In vehicular applications, such as fuel cell power plants providing electricity for electric vehicles, the use of two side-by-side fuel cell stacks may provide the best physical fit within the vehicle. In such a case, fuel maldistributions may result from variations in fuel distribution to the respective stacks.
Pass-to-pass leakage variations can be eliminated by using single pass fuel flow field arrangements.
It is known that recycling a large amount of fuel, on the order of 15-25% at maximum power output, will allow an overall power plant fuel utilization which is significantly higher than the utilization in a single pass through the stack. However, the fuel that is recycled typically has to have sufficient pressure to overcome the fuel flow field pressure drop, which requires a pump or compressor of some sort, in the usual case. Due to the low density of hydrogen, positive displacement pumps and centrifugal blowers are typically utilized for significant recycle flows. These are required to operate at very high speeds (in excess of 20,000 rpm), are noisy and are unreliable. Other forms of impellers, such as electrochemical hydrogen pumps and ejectors, have heretofore been incapable of use across a wide range of power output operating conditions.